Prochordates are the forerunners of Vertebrates. They do not have a cranium or skull. So they are called Acrania. The classification is based on the nature of the notochord. The following are the three subphyla of prochordata.
(a) Hemichordates:
Eg- Balanoglossus. The organism lack notochord and are without backbones. They are tuberculous forms. The body is soft, vermiform, unsegmented, bilaterally symmetrical and triploblastic. The notochord is persistent as the stomochord in the anterior region.
(b) Cephalochordates:
Eg- Amphioxus: The notochord extends forward beyond the brain. Small fish like marine chordates with i unpaired dorsal fins.
(c) Urochordates:
Eg- Ascidians: The notochord is confined to the tail region of the larva. In adults, they degenerate and are in sessile forms. The body is enveloped by a tunic or test.
